The present invention relates to thermally assisted magnetic recording media and a magnetic recoding and reproducing apparatus (HDD) using the same.
With the improvement in processing speed of a computer in recent years, high speed and high densification are demanded of the magnetic recording and reproducing apparatus (such as, HDD) which records and reproduces information and data. However, there is a physical limitation in the densification of CoCrPt group media currently being used.
In the case of HDD apparatus, the magnetic recording medium by which information is recorded has a magnetic layer containing the aggregate of fine magnetic particles. In order to perform high density record, it is necessary to make a magnetic domain recorded on the magnetic layer small. To be able to classify a small record magnetic domain, it is required to make the boundary of a magnetic domain smooth, and to micrify the magnetic particle contained in the magnetic layer. Moreover, the boundary of a magnetic domain is shaken if a magnetization reversal carries out in a chain to adjoining magnetic particles. Thus, the magnetic particles need to be magnetically segmentalized by a non-magnetic material (this is commonly called the ‘gain segregation’ technique). Moreover, from a viewpoint of the magnetic interaction between head media, for recording high density, it is also necessary to make the thickness of a magnetic layer small.
Therefore, for high-density recording, the volume of the magnetization reversal unit (almost equal to a magnetic particle) in a magnetic layer needs to be made even smaller. However, when the magnetization reversal unit is micrified, its anisotropy energy [magnetocrystalline anisotropy energy constant (KF)×volume of a magnetic particle VF, KFVF, (F is an abbreviation of Ferromagnetism)] becomes smaller than the thermal fluctuation energy [Boltzmann constant (KB)×temperature (T)] so that the magnetic domain cannot be maintained. This phenomenon is called thermal fluctuation, which is the main factor of the physical limitation (also called a heat fluctuation limitation) of recording density.
To prevent the magnetization reversal due to thermal fluctuation, the magnetocrystalline anisotropy energy constant KF may be increased. However, in case of the HDD medium, the coercive force HC during the magnetization reversal operation at high speed is almost proportional to KF, and recording cannot be done at a magnetic field (maximum 10 kOe) where recording head may be generated. Alternatively, VF may be increased to prevent the magnetization reversal due to thermal fluctuation. However, if VF is increased by increasing the size of magnetic particles on the medium surface, high-density recording cannot be attained. Meanwhile, if VF is increased by increasing the thickness of a recording layer, the head magnetic field does not reach sufficiently to the bottom of the recording layer and as a result, the magnetization reversal does not generate, making high-density recording impossible.
In order to solve the above problem, an idea called heat assistant magnetic recording is proposed. This performs magnetic recording by heating a recording layer made of a material of large KF and locally reducing KF (i.e., HC). By this method, even if KF of the recording layer is large under the operating environment of a medium (usually room temperature (RT)), the magnetization reversal becomes possible in the record field generated with the present head.
However, since an adjoining track is somewhat heated at the time of record, the phenomenon (cross erase) in which thermal fluctuation is accelerated by adjoining track and a record magnetic domain is eliminated may happen. Moreover, since the medium is still warm to some extent even when a head magnetic field is lost immediately after recording, the thermal fluctuation is accelerated and a magnetic domain already formed may still disappear. To resolve these problems, it is necessary to use ingredients featuring a high sensitivity to the change in the temperature of KF (i.e., HC) around the recording temperature. However, since temperature change of KF (i.e., HC) of the current CoCrPt group medium is almost linear in general, the above-mentioned conditions cannot be fulfilled.
As an attempt to solve the foregoing problem, Japanese Patent Application Laid-Open Pub. No. 2002-358616 disclosed a medium structure consisting of “Functional layer (under layer)/Switching layer (intermediate layer)/Recording layer (upper layer).” According to the disclosure, the functional layer contains a ferrimagnetic (F) layer (since ferrimagnetic substance belongs to ferromagnetic substance), the ferrimagnetic substance is also abbreviated as (F) formed of amorphous rare earth (RE.)-transition metal (TM.) alloy such as TbFe, the recording layer is formed of a ferromagnetic (F) layer of the current CoCrPt group, and the switching layer is formed of a ferrimagnetic (F) layer of RE.-TM. alloy, in which the F layer has a Curie point (TC) just below the recording temperature (TW). In this medium with the “Functional layer/Switching layer/Recording layer” structure, “F/F/F” exchange interaction is formed at RT. Owing to this, the KF (i.e., HC) value at RT can be increased to a large value, which in turn makes it possible to enhance resistance against thermal fluctuation. As mentioned before, since the exchange coupling of “F/F/F” disappears at TC of the intermediate layer, and becomes magnetic state of “F/Para./F” (Para. is the abbreviation of paramagnetism), the KF (i.e., HC) value of the recording layer is rapidly lowered to the value of a single recording layer at the temperature of TC of the intermediate layer. Accordingly, a rapid temperature variation of KF (i.e., HC) is obtained around the temperature TC of the intermediate layer (the change of the temperature of KF (i.e., HC), dKF/dT or dHC/dT toward the temperature T is defined as “temperature gradient of KF” or “temperature gradient of HC”, and these expressions will also be used throughout the specification). Since the KF (i.e., HC) value is rapidly lowered to the value of a single recording layer at the temperature TW, write operation can be carried out on the recording layer under a small recording magnetic field.
Moreover, Appl. Phys. Lett., Vol. 82, pp. 2859-2861(2003) disclosed a magnetic film composed of “FeRh (under layer)/FePt (upper layer).” FeRh group material is the only one that goes through phase transition from antiferromagnetism (AF) to ferromagnetism (F) around 100° C. According to this article, at RT, HC of FePt can be enhanced by the exchange coupling of “AF/F.” Also, as the phase is transited from AF→F, the exchange coupling of “AF/F” disappears, and the exchange coupling of “F/F” is generated instead. Since F in the under layer FeRh has a soft magnetic property, a drastic temperature variation in HC is obtained around AF→F phase transition temperature (about 100° C.) of FeRh.